Dragon's Den originally premiered on BBC Two.
The British programme began broadcasting on BBC Two in January 2005. BBC Two was the BBC channel traditionally associated with factual, documentary and specialist entertainment programming, making it the original home for a show centred on entrepreneurs, investment and business plans.
The programme's success helped it become one of the BBC's best-known factual-entertainment formats. Although its scheduling and presentation have evolved, its original association with BBC Two remains an important part of its broadcast history. The title is styled Dragon's Den in the UK, with the plural possessive form used in the programme branding.
BBC One is the BBC's main general-entertainment channel, so it is a plausible but incorrect answer. BBC Three and BBC Four also launched or developed later identities and were not the channel on which the British series premiered.